The present invention relates to an electrode terminal structure for a piezoelectric film element giving a good and tenacious bonding structure.
Recently, a piezoelectric film or sheet element comprising a single layer or plural layers of polymer piezoelectric film of, for example, a vinylidene fluoride resin, and a pair of electrode films disposed on both sides of the polymer piezoelectric film, has been widely used as, e.g., an electromechanical transducer element for use in vibration sensors, microphones, speakers, etc. (Herein, the term "piezoelectric element" refers to the above-mentioned structure and is meant to also cover such a structure which is used functionally as a pyroelectric element.) For example, conventional vibration sensors used heretofore have been those obtained by forming electrode films on both sides of a ceramic piezoelectric sheet of PZT, etc., and soldering lead wires to respective electrode films. However, these ceramic vibration sensors have several difficulties as follows. Thus, a ceramic vibration sensor does not fit a curved surface so that the accuracy of the measurement is lowered. Moreover, the measurable vibration frequency of such sensor is limited to 500 Hz or below, and it cannot be applied to measurement of a high frequency vibration reaching as high as 10 kHz or so as observed, e.g, in turbines. In contrast thereto, sensors using polymer piezoelectric elements are free from these difficulties and considered to be essentially suited.
However, there is a serious problem in using a polymer piezoelectric element as an electromechanical coupling agent that a structure of taking out lead wires from the electrodes disposed thereon is generally fragile and is not durable in continuous or successive use under vibration. For example, as one of the most fundamental ideas, it may be conceivable to solder lead wires to electrode films formed on a polymer piezoelectric film as has been explained with reference to the ceramic vibration sensor. This is, however, not appropriate because the polymer piezoelectric film per se is deteriorated due to heat. It is also conceivable to use an electroconductive bonding agent obtained by dispersing metal powder, carbon powder, etc., into various kinds of resinous binders to bond the lead wires to the electrode films. However, such an electroconductive bonding agent is relatively fragile and cannot possibly be durable against successive vibration applied in use as an electromechanical transducer element.